A press forming machine is also used for punching press, drawing, stamp forging, and injection molding. A press forming machine is generally used in which one mold is fixed and the other mold is movable. A vertical press forming machine includes a lower fixed plate, a plurality of supports supported by the lower fixed plate, an upper support plate held by the supports and a pressure plate capable of reciprocating along the supports between the lower fixed plate and the upper support plate and having a forming space between the pressure plate and the lower fixed plate. A fixed mold is mounted on the lower fixed plate and a movable mold is set to the downside of the pressure plate in the forming space and a workpiece is formed between the fixed mold and the movable mold. The pressure plate is normally planar and vertically moved by a driving mechanism. It is preferable to press-form, while keeping the movable mold at a desired positional relationship with the fixed mold, for example, to press-form by moving the movable mold while keeping it horizontal. Therefore, the pressure plate is moved while being kept horizontally. The support is built so as to be thick and have a rigidity in order to prevent the pressure plate from tilting during the press-formation. However, the pressure plate or the like is bent and a tilt occurs due to the clearance of a slide portion in some cases. Therefore, it is necessary to correct a mold in order to prevent the tilt.
Moreover, because a workpiece formed through press forming has a complex shape such as a three-dimensional shape, it is found that not only the magnification of a force applied to the pressure plate is changed in accordance with progress of press-formation but also the position to which the force is applied moves in accordance with the press-formation.
When a vertical resultant force of forces working on the pressure plate is applied to the central position of the pressure plate, an angular moment for tilting the pressure plate is not created to the pressure plate. But, since the position at which the force works is moved as described above, the position and the magnitude of the angular moment are changed. Therefore, deformations of various portions of a press forming machine such as an elongation and a warp of the pressure plate, the upper support plate and the fixed plate which occur during the press-formation are changed in accordance with progress of the press-forming.
Because a descending progress of the pressure plate is changed due to a load applied to the pressure plate or deformation of the press forming machine due to the load, the positional relationship between the fixed mold and the movable mold or the pressure plate may not be horizontal. Therefore, the present inventors improved a press forming machine having a plurality of driving sources for driving a pressure plate and proposed a press forming machine capable of keeping a pressure plate horizontal by controlling the driving sources in Japanese Patent Laid-Open No. 2002-263900. In the proposed press forming machine, a pressure plate is kept horizontal by supplying a driving pulse signal having a frequency higher than a predetermined frequency to a driving source (servomotor) set to a position close to a portion whose progress is delayed on the pressure plate and supplying a driving pulse signal having a frequency lower than the predetermined frequency to a driving source whose progress is relatively advanced. However, it is found that when an overload occurs in a driving source present at the central portion of the pressure plate, a phenomenon in which the above adjustment cannot be made occurs.
In the above proposed press forming machine, when having three or more pressure points on the pressure plate among which a pressure point present at the central portion is surrounded by the pressure points present on the periphery, a driving source for driving a driving shaft set to the pressure point at the central portion may be overloaded. When forming a workpiece by holding a forming mold between the pressure plate and a fixed plate, a load larger than the load at peripheral portion is applied to the central portion of the pressure plate. Therefore, the displacement of the central portion is most delayed. Therefore, more driving pulse signals are supplied to the driving source for driving the central driving shaft, and displacements of the central portion and peripheral portion of the pressure plate are equalized to keep their horizontal state. However, the driving shaft set in the center of the pressure plate is applied to by a load larger than that applied to each of a plurality of driving shafts present at the peripheral portion, since part of a load applied to each of the driving shafts on the periphery works on the central driving shaft and a total load is applied to the central driving shaft. Therefore, it is estimated that the driving source for driving the central driving shaft is overloaded.